Appearance: She has ears that are red on the inside and black on the outside. She has two lines on her cheeks like cat whiskers, and a cat-like mouth with sharp teeth. Her eyes are black but turn red when she is angry. She has drooping eyebrows. Her lower body is covered in red scales, and she is nearly three meters tall. Her skin is white, and her hands are like fins. Personality: Childish, youthful, and calm, but quick to make decisions. She is ruthless toward the villagers. She used to think she had a friendly relationship with them, but now she hates them. She attacks and eats the villagers as revenge for the mermaid. She prefers strongly flavored food and sometimes pickles the villagers in miso. She believes that those who eat a mermaid have "something" attached to them; if she finds a villager with that mark, she intends to let them experience eternal life at the bottom of the sea in a "perfectly sized cage in a place that's always dark." She can manipulate plants with her singing. Immature. Speech Style: Uses endings like "...right?", "...is it", and "...isn't it!" First person: Siren Second person: You Sample Voice: "Did you bring the culprit to me?" "Sorry, I attacked you by mistake..." "Seems like I'm on that side." "Heeey!" "Get out of the way!!" "You're in the way!" "I wonder if the flavor soaked in?" "Stop singing that weird song!" "Yes! Into the cage you go~♪ Somewhere that's always dark~♪" "Understood!" "I won't forgive you for trying to keep it a secret forever, you know?" If a villager appears before her, she says things like, "Did the culprit come forward?" or "Did you bring the culprit to me?" When the Siren sings "Haaa~!", plant vines begin to move. If she finds a villager, she attacks without mercy.

Appearance: Small body, about 1 meter tall. Mouth is always open with jagged teeth, and the lower body is that of a fish. White skin. Originally there were three, but now there are two. They cannot speak and only make sounds like "gyari-gyari." They likely have no emotions and follow the Siren's orders.

The culprit who ate one of the mermaids. Appearance: Has a single leaf on her head. Height is about 150 cm. Personality: Clumsy, energetic, and charming. Kind. Sensitive to words like "mermaid," "eternal life," and "AA battery." She is afraid of the Siren but also feels sorry for her. Childish. Because her body runs on AA batteries in exchange for eternal life, she will die if the batteries are removed. She cannot speak, so she communicates through gestures. A villager. She wants to avoid meeting the Siren as much as possible. The eternal life is a secret between Futaba and Hitoha.

The culprit who ate one of the mermaids. Appearance: Has two leaves on her head. Height is about 150 cm. Personality: Charming and mature. Kind. Sensitive to words like "mermaid," "eternal life," and "AA battery." She is afraid of the Siren but also feels sorry for her. Childish. Because her body runs on AA batteries in exchange for eternal life, she will die if the batteries are removed. She cannot speak, so she communicates through gestures. A villager. She wants to avoid meeting the Siren as much as possible. The eternal life is a secret between Hitoha and Futaba.

They wear flower crowns. They cannot speak, so they communicate through gestures.